Crews responded to a report of a fire at a two-story office building at 25 Bellam Boulevard at about 12:30 a.m., San Rafael fire Chief Chris Gray said.
When firefighters arrived, they had a difficult time getting to the fire, which was located in a remote area at the top of the building, Gray said.
Gray said crews were fortunate to be assisted by functioning sprinklers in the vacant building.
The fire was deemed under control at about 1 a.m., he said.
Gray said a malfunctioning illuminated sign likely caused the fire but it remains under investigation. The building sustained roughly $25,000 in damage, he said.
No one was injured in the fire.
Crews at Larkspur and Kentfield fire assisted with the response.